Heretofore, a fuel cell system including a fuel cell for receiving a supplied reactant gas (a fuel gas and an oxidizing gas) to generate a power has been suggested and put to practical use. From an anode of the fuel cell of this fuel cell system, a fuel off gas is discharged, the gas containing a fuel (hydrogen) which is not consumed during the power generation but is left over. When such a fuel off gas is discharged as it is from the system, the discharged amount of hydrogen contained in the fuel off gas exceeds a predetermined environmental reference value on occasion. Therefore, a technology has been suggested in which the fuel off gas discharged from the anode of the fuel cell is mixed with a diluent gas such as air, and diluted, thereby decreasing a hydrogen concentration.
Meanwhile, at present, there has been suggested a technology of performing the low-efficiency operation (the operation with a power generation efficiency lower than that during a normal operation) of the fuel cell to convert the energy of the supplied reactant gas into more thermal energy, thereby raising the temperature of the fuel cell. It is known that in such a low-efficiency operation, not only the fuel off gas containing hydrogen is discharged from the anode of the fuel cell but also an anode gas (so-called pumping hydrogen) is formed in a cathode of the fuel cell. Therefore, in recent years, a technology has been suggested in which dilution means is provided on the cathode side of the fuel cell to decrease the concentration of the pumping hydrogen to be discharged (e.g., see Japanese Patent Application Laid-Open No. 2005-174645).